Ferromagnetic intermetallic compounds and method of preparation



United States Patent 3,326,637 FERROMAGNETIC INTERMETALLIC COM- POUNDSAND METHOD 0F PREPARATION Frederic Holtzberg, Pound Ridge, and SiegfriedJ. Methfessel, Montrose, N.Y., assignors to International BusinessMachines Corporation, New York, N.Y., a corporation of New York NoDrawing. Filed Dec. 27, 1963, Ser. No. 334,023

11 Claims. (Cl. 23-404) This application is a continuation-impart ofU.S. patent application Ser. No. 302,708, filed Aug. 16, 1963, nowabandoned, entitled Ferromagnetic Intermetallic Compounds and Method ofPreparation, by F. Holtzberg et al.

This invention relates to new rare earth intermetallic compounds and,more particularly, to compounds having the formula A M wherein A is arare earth selected from the group consisting of Gd, Tb, Dy, and Ho; andM is a transition metal selected from the group consisting of Pd and Ptand their preparation.

When A is Gd, the compounds Gd Pd and Gd Pt are ferromagnetic below atemperature of T =61 C. which is a. higher ferromagnetic Curietemperature than for the pure Gd metal (T =l6 C.). When A is Tb, Dy, orH0, the compounds Tb Pd Dy Pd Ho Pd Tb Pt Dy 'Pt or Ho Pt first becomemetamagnetic and then ferromagnetic at temperatures between liquidnitrogen and liquid helium temperatures.

The rare earth metals and their compounds are important magneticmaterials because they generally exhibit higher magnetic moments thanthe iron group metals (e.g., Fe,'Co, and Ni) and their compounds.

The magnetic moment of the rare earth elements is either the sum or thedifference of the spin and orbital moments of the unpaired electrons inthe 4f shell, the difference resulting for the lighter and the sum forthe heavier elements. The outer bonding orbitals effectively shield the4f shell so that chemical bond formation has little effect on the totalmagnetic moment. In contrast, the unpaired 3d electrons of the irongroup metals are directly involved in bond formation and magneticcoupling so that the compounds and alloys of these elements generallyhave different moments.

The Curie temperatures for the rare earth elements, however, arerelatively low, e.g., Dy, T =80 K. and Gd, T =290 K. As a consequence,the practical application of these elements as magnetic materials islimited to low temperature systems such as cryogenic systems. Thepossibility of increasing the Curie temperature of rare earth metals byalloying with other elements increases the range of their practicalapplicability in magnetic and electronic devices such as transformersand relays for which a high permeability is desired.

Heretofore, most investigations of the rare earth-palladium and rareearth-platinum systems have been confined to rare earth concentrationsless than 50% and to compounds of the type AM (Where A is a rare earthelement and M is a Pd or Pt) which have been described structurally (A.E. Dwight, J. W. Downey, and R. A. Conner, In, Some A13 Compounds of theTransition Metals, acta Cryst, 14, 75, 1961). In the previousinvestigations, there has been no discussion of the magnetic propertiesof these compounds.

. It is an object of 'the invention to prepare rare earth intermetalliccompounds.

It is another object of the invention to prepare new rare earthintermetallic compounds which are ferromagnetic.

It is a further object of the invention to prepare rare earthintermetallic compounds having the formula A M wherein A is a rare earthselected from the group consisting of Gd, Tb, Dy, and Ho and M is atransition metal selected from the group consisting of Pd and Pt.

Another object of the invention is to prepare a rare earth intermetalliccompound having the formula Gd Pd [71.4 at.(78.7 wt.) percent Gd] Astill further object of the invention is to prepare a rare earthintermetallic compound having the formula Tb Pd [71.4 at.(78.9wt.)percent Tb] Yet another object of the invention is to prepare a rareearth intermetallic compound having the formula Dy 'Pd [71.4 at.(79.2wt.) percent Dy] Still another object of the invention is to prepare arare earth intermetallic compound having the formula Ho Pd [71.4at.(79.6 wt.)percent Ho] A further object of the invention is to preparea rare earth intermetallic compound having the formula Gd Pt [71.4at.(66.8 wt.)percent Gd] Further another object of the invention is toprepare a rare earth intermetallic compound having the formula Tb Pt[71.4 at.(67.2 wt.)percent Tb] Further still another object of theinvention is to prepare a rare earth intermetallic compound having theformula Dy Pt [71.4 at.(67.6 wt.)percent Dy] Another further object ofthe invention is to prepare a rare earth intermetallic compound havingthe formula Ho5Pt [71.4 at.(67.9 wt.)percent Ho] The foregoing and otherobjects, features, and advantages of the invention will become apparentfrom the more particular description of a preferred embodiment of theinvention.

The new rare earth intermetallic compounds disclosed herein have theformula A M wherein A is a rare earth selected from the group consistingof Gd, Tb, Dy, and Ho and M is a transition metal selected from thegroup consisting of Pd and Pt. The following rare earth intermetalliccompounds are examples of the invention:

Gd Pd [71.4 at.(78.7 wt.) percent Gd] Tb Pd [71.4 at.(78.9 wt.)percentTb] Dy Pd [71.4 at.(79.2 wt.)percent Dy] Ho Pd [71.4 at.(79.6wt.)percent Ho] 'Gd Pt [71.4 at-.(66.8 wt.)percent Gd] Tb Pt [71.4at.(67.2 wt.)percent Tb] Dy Pt [71.4 at.(67.6 wt.)percent Dy] Ho Pt[71.4 at.(67.9 wt.)percent Ho] These rare earth intermetallic compoundshaving the formula A M are prepared by mixing appropriate amounts (5moles) of the rare earth element selected from the group consisting ofGd, Tb, Dy, and Ho and 2 moles of the transition metal selected from thegroup consisting of Pd and Pt of the component elements in finelydivided form and then heating to melt and react the component elements.The heating is accomplished by using one or theother followingmetallurgical procedures.

One procedure involves placing the sample mixture in an inert refractorymetal crucible (e.g., tantalum, molybdenum, etc.) which in turn isevacuated and sealed (e.g., by cold welding). The crucible is now placedin a quartz vacuum system centered in a radio frequency inductionheating coil. An ambient atmosphere of helium or argon is often used inplace of the vacuum. Power is delivered to the coil at a rate such thatthe crucible temperature is raised to a temperature between 1400" C. to1600 C.

This temperature is maintained until the reaction is completed. Thepower is now turned off and the crucible rapidly cools to roomtemperature. Since there is a finite solubility of tantalum in the melt,the following are melting process of preparing these compounds ispreferred.

The other and preferred procedures used in preparing the rare earthintermetallic compound disclosed in the invention is an arc meltingprocedure. Appropriate quantities of the rare earth and palladium infinely divided form are mixed in amounts corresponding to those desiredin the final compounds and pressed into a pellet for convenienthandling. This pellet is then placed in an arc furnace of a commerciallyavailable type (e.g., Model AF92 MRC Manufacturing Corp. or one similarto that described in FIGURE 3 of US. Patent No. 2,989,480). The furnacechamber is evacuated and flushed with an inert gas, such as argon, neon,krypton, xenon, or helium, three times to purge the chamber. In such afurnace, a movable cathode is used to strike an arc to the water-cooledcopper hearth (anode) which contains several wells or shallowdepressions to hold the metals being melted and reacted. The arc isstruck to the anode in the vicinity of the reactants, which are fused(melted) by the heat of the arc. Since the cathode is mounted through aball and swivel joint, the molten material can be stirred by precessingthe cathode tip around the periphery of the melt.

After the sample has been fused, the arc is interrupted and thesolidified melt is turned over in the well then remelted in the arc. Byrepeated turnings and melting, homogeneity in the sample can beachieved. Temperatures in excess of 3500 C. can readily be generated.Such a temperature is more than sufficient to fuse the metals Gd, Tb,Dy, Ho, and Pd; however, the cooled anode keeps a thin layer of thematerials being fused in a solid condition, on the cold anode surface,so that the melt itself does not ever contact the metal of the anode.Alloying of the anode and the melt is thus avoided.

These rare earth intermetallic compounds are brittle metallic materialswhich form a protective oxide coating in ambient atmospheres.

The compounds have the following magnetic properties; Gd Pd isferromagnetic below 61 C. with a saturation moment of 197 emu per gramin agreement with the moment calculated for an atomic moment of 7 Bohrmagnetons per Gd atom. The saturation magnetization decreases withincreasing temperature, T, according to the (T) law up to T=0.8T TheCurie temperature T where the magnetic moment disappears is at 334 K. GdPd is a soft magnetic material with a coercive force less than 100oersteds and the magnetization o' saturates at constant temperature Twith a field H to the saturation value em following the law H, T co,

with a magnetic hardness a= H= 110 oersteds oo T which value is lowerthan a Gd=359 oe. for pure gadolinium metal.

Since the Gd Pd has a coercive force and hardness lower than Gd metaland a saturation magnetization higher than most iron group metals andtheir alloys (approximately 25,000 gauss at 0 K. compared with theapproximate value for iron of 21,000 gauss) thus the Gd Pd is useful inelectronic and magnetic devices such as transformers or relays incomputer circuitry. At room temperature (20 C.) Gd Pd has a magneticmoment of 45% of the saturation value available. (Gd is paramagnetic atthis temperature.) Since the Curie temperature is at 61 C., and thevariation of the magnetization with temperature is high around roomtemperature, Gd Pd is used as core material in thermal switching,control, and safety devices.

In contrast to the Gd Pd the compounds Tb Pd Dy Pd and Ho Pd havepararnagnetic-metalmagnetic transitions (Neel points T below liquidnitrogen temperature (e.g., the T for Dy Pd is 41 K.; Tb Pd is 62 K. andHo Pd is 33 K.), and become ferromagnetic at still lower Curietemperatures (e.g., Dy Pd T =25 K.; Tb Pd T ==30 K.; and Ho Pd T =10 K.)with a high coercive force (e.g., the H for Tb Pd is 12,800 oe.; Dy Pdis 9700 oe. and Ho Pd is 1300 oe.) The extreme magnetic hardness makesthese compounds useful as permanent magnets in cryogenic circuits anddevices operating around liquid helium temperature (4.2" K.) which isthe normal operating temperature for cryogenic circuitry.

The similarity in chemical properties of Pd and Pt refiects in thesimilarity in magnetic properties of A Pd and A Pt for example, Gd Pdand Gd Pt are isostructural and have the same ferromagnetic Curietemperature.

Example I.Gd Pd [71.4 at. (78.7 wt.) percent Gd] 7.87 grams of Gd metalfilings and 2.13 grams of Pd powder are thoroughly mixed and pressedinto pellets which are placed in a tantalum crucible which is evacuatedand sealed by cold welding. The crucible is placed on a pedestal in anevacuated quartz cylinder and heated to 1600 C. for 2 minutes with aradio frequency induction heating coil and rapidly cooled to roomtemperature. The resulting product is Gd Pd Example II.Gd Pd [71.4 at.(78.7 wt.) percent Gd] 7.87 grams of Gd metal filings and 2.13 grams ofPd powder are mixed and pressed into a pellet. The pellet is placed in awell of the water-cooled copper hearth of an arce melting furnace. Thefurnace chamber is evacuated and flushed with argon gas three times topurge the chamber of reactive gases. An arc is struck between thetungsten cathode and the water-cooled hearth of the arc furnace. Acurrent of amps at 40 volts liquified the sample in about 10 seconds.The melt is stirred by precessing the cathode around the periphery ofthe melt for about 30 seconds, and then the power is turned off and themelted sample allowed to solidify and cool. The sample is then invertedand the above are melting, cooling and turning procedure is repeateduntil a homogeneous product is obtained. Since the sample is in directcontact with the cold copper hearth, the cooling is essentially a quenchfrom the molten state. Micrometallurgical examination revealed thecompleteness of the reaction by showing a single phase compound. Thiscompound is Gd Pd Example III.--Tb Pd [71.4 at. (78.9 wt.) percent Tb]The process of Example II is repeated except that 7.89 grams of Tb and2.11 grams of Pd are used instead of the amounts of the Gd and Pd ofExample 11. The resulting compound is Tb Pd Example I V.Dy Pd [71.4 at.(79.2 wt.) percent Dy] The process of Example II is repeated except that7.92 grams of Dy and 2.08 grams of Pd are substituted for the amounts ofGd and Pd of Example II. The resulting compound is Dy Pd Example V.-HoPd [71.4 at. (79.6 wt.) percent Ho] The procedure of Example 11 isrepeated except that 7.96 grams of Ho and 2.04 grams of Pd aresubstituted for the amounts of Gd and Pd used in Example II. Theresulting product is Ho Pd Example VI.Gd Pt [71.4 at. (66.8 wt.) percentGd] of the arc furnace. A current of 150 amperes at 40 volts liquifiedthe sample in about minutes. The melt is stirred by precessing thecathode around the periphery of the melt for about 30 seconds, and thenthe power is turned off and the melted sample allowed to solidify andcool. The sample is then inverted and the above arc-melting, cooling,and turning procedure is repeated until a homogeneous product isobtained. Since the sample is in direct contact with the cold copperhearth, the cooling is essentially a quench from the molten state.Micrometallurgical examination revealed the completeness of the reactionby showing a single phase compound. This compound is Gd Pt ExampleVII.Tb Pt [71.4 at. (67.2 wt.) percent Tb] The process of Example VI isrepeated except that 6.72 grams of Tb and 3.28 grams of Pt are usedinstead of the amounts of the Gd and Pt of Example VI. The resultingcompound is Tb Pt Example VIIl.-Dy Pt [71.4 at. (67.6 wt.) percent Dy]The process of Example VI is repeated except that 6.75 grams of Dy and3.24 grams of Pt are substituted for the amounts of Gd and Pt of ExampleVI. The resulting compound is Dy Pt- Example IX.-Ho Pt [71.4 at. (67.9wt.) percent Ho] The procedure of Example VI is repeated except that6.79 grams of Ho and 3.21 grams of Pt are substituted for the amounts ofGd and Pt used in Example VI. The resulting product is Ho Pt Rare earthintermetallic compounds having the formula A M (wherein A is selectedfrom the group consisting of the Gd, Tb, Dy, and Ho; and M is atransition metal selected from the group consisting of Pd and Pt) havebeen prepared and found to be ferromagnetic.

While the invention has been particularly shown and described withreference to a preferred embodiment thereof, it will be understood bythose skilled in the art that the foregoing and other changes in formand details may be made therein without departing from the spirit andscope of the invention.

What is claimed is:

1. A rare earth intermetallic compound having the formula A M wherein Ais a rare earth selected from the group consisting of Gd, Tb, Dy, andHo, and M is a transition metal selected from the group consisting of Pdand Pt.

2. The rare earth intermetallic compound Gd Pd [71.4 at. (78.7 wt.)percent Gd] 3. The rare earth intermetallic compound Tb Pd [71.4 at.(78.9 wt.) percent Tb] 4. The rare earth intermetallic compound Dy Pd[71.4 at. (79.2 wt.) percent Dy] 5. The rare earth intermetalliccompound Ho Pd [71.4 at. (79.6 wt.) percent Ho] 6. The rare earthintermetallic compound Gd Pt [71.4 at. (66.8 wt.) percent Gd] 7. Therare earth intermetallic compound Tb Pt [71.4 at. (67.2 wt.) percent Th]8. The rare earth intermetallic compound Dy Pt [71.4 at. (67.6 Wt.)percent Dy] 9. The rare earth intermetallic compound Ho Pt [71.4 at.(67.9 wt.) percent Ho] 10. The process of preparing a rare earthintermetallic compound having the formula A M wherein A is a rare earthselected from the group consisting of Gd, Tb, Dy and Ho, and M is atransition metal selected from the group consisting of Pd and Pt whichcomprises:

(1) mixing together in finely divided form A and M in proportions suchthat a rare earth compound produced by heating has the above formula;

(2) are melting the thus formed mixture in an inert atmosphere on a coldcopper hearth;

(3) turning the thus formed melt and again arc melting in an inertambient atmosphere and then cooling to room temperature;

(4) repeating steps 2 and 3 a plurality of times; and

(5) cooling rapidly to room temperature.

11. The process of preparing a rare earth intermetallic compound havingthe formula A -M wherein A is a rare earth selected from the groupconsisting of Gd, Tb, Dy and Ho, and M is a transition element selectedfrom the group consisting of Pd and Pt which comprises:

(1) mixing together in finely divided form A and M in a 5:2 molar ratio;

(2) are melting four times the thus formed sample mixture in an argonatmosphere on a copper hearth and turning the sample mixture betweeneach melting; and

(3) cooling rapidly to room temperature.

References Cited UNITED STATES PATENTS 8/1963 Wallace et al -152 FOREIGNPATENTS 649,838 10/1962 Canada.

OTHER REFERENCES Mellor: Comprehensive Treatise on Inorganic andTheoretical Chemistry, vol. 16, p. 211 (1937).

OSCAR R. VERTIZ, Primary Examiner. H. S. MILLER, Assistant Examiner.

1. A RARE EARTH INTERMETALLIC COMPOUND HAVING THE FORMULA A5M2 WHEREIN AIS A RARE EARTH SELECTED FROM THE GROUP CONSISTING OF GD, TB, DY, ANDHO, AND M IS A TRANSITION METAL SELECTED FROM THE GROUP CONSISTING OF PDAND PT.
 10. THE PROCESS OF PREPARING A RARE EARTH INTERMETALLIC COMPOUNDHAVING THE FORMULA A5M2 WHEREIN A IS A RARE EARTH SELECTED FROM THEGROUP CONSISTING OF GD, TB, DY AND HO, AND M IS A TRANSITION METALSELECTED FROM THE GROUP CONSISTING OF PD AND PT WHICH COMPRISES: (1)MIXING TOGETHER IN FINELY DIVIDED FORM A AND M IN PROPORTIONS SUCH THATA RARE EARTH COMPOUND PRODUCED BY HEATING HAS THE ABOVE FORMULA; (2) ARCMELTING THE THUS FORMED MIXTURE IN AN INERT ATMOSPHERE ON A COLD COPPERHEARTH; (3) TURNING THE THUS FORMED MELT AND AGAIN ARC MELTING IN ANINERT AMBIENT ATMOSPHERE AND THEN COOLING TO ROOM TEMPERATURE; (4)REPEATING STEPS 2 AND 3 A PLURALITY OF TIMES; AND (5) COOLING RAPIDLY TOROOM TEMPERATURE.